This invention relates to a method, a controller and a system for controlling slug flow of a multiphase fluid comprising gas and liquid in a conduit, such as a riser pipeline, the pipeline being provided with a control valve with a variable aperture which enables the conduit to be constricted. The method, controller and system may be arranged to prevent the excessive growth of slug flow by choking the flow by varying the aperture of the valve.
There are many applications in which fluid is required to flow through conduits which include riser pipelines, but perhaps one of the most challenging applications is the extraction of fluid under relatively low pressures from wells in the gas and oil industry. In the case of wells located on a seabed there may be a considerable rise of the fluid from the base of the riser to the processing apparatus on the sea surface. Low pressures are typically present where a well is nearing the end of its useful life, a so called brown oil field, and under such conditions it is common for the flow of fluid in a riser pipeline to give rise to slug flow.
Slug flow is troublesome to offshore oil production systems, because it leads to sudden and generally cyclical surges in the flow of liquid (e.g. oil) and gas from the riser pipeline to subsequent processing apparatus located topside of the pipeline, e.g. downstream of the top of the pipeline. Because of the problems of slug flow, wells are often considered to have reached the end of their useful life sometime before they are exhausted simply because of the low pressures which give rise to troublesome slug flows.
Many attempts have been made to prevent the formation of slug flow in order to make it viable to extract from low pressure wells (the problem also occurs with high pressure wells but is less prominent). Known solutions focus on active control of the flow by opening and closing a control valve, or choke valve, located at the topside of the riser pipeline. This has been shown to be effective in preventing the formation of severe slugs but it inevitably reduces the overall flow rate through the pipeline when measured over an extended period of time. Since the profitability of the system depends on getting out as much oil and gas as quickly as possible this loss of flow is disadvantageous. Furthermore, many current systems of flow control using choking rely upon the measurement of pressure from the base of the riser being available. This is unsatisfactory as it is expensive to install sensing devices at the riser base, especially in deep seas, and there are obvious difficulties with maintaining the sensors in such a location. For hydrodynamic slugs, due to the small size of slugs, the riser base pressure can only provide an average measurement over many slugs in the riser, hence the instability in the system cannot properly be observed.